I'm not sure if RNDIS or USB-ECM drivers exist for OS9, but if they do, you could probably skip the ethernet adapter and just connect the phone directly for tethering.
well, thats the first solution I would try. What iBook G3 is it, Clamshell or white? Both have USB1.1, so a USB wifi stick should work under Tiger.
Edimax ew-7811un USB Wi-Fi adapter, the first generation model of it will do WPA2.
I bought a Netgear Universal N300 adapter back when they were still being made. They were like $20. Now they're over $200. :/
I liked it because the case looked like a miniature PowerBook G3. It's also powered by USB. It looks like an official accessory to the PowerBook. It's also really small and light.
I had a lone USB wifi dongle that worked in 10.4 for my 1st gen iMac, but after I reinstalled 10.4 myself, I could never get the drivers happy again . The guy I bought it from had long forgotten how he got it sorted, and I've long since given up. Would have been / would be nice though.
It's the Snow White iBook G3 800 MHz. A pretty nice machine actually, and the screen still looks great. It's twice as thick as my MacBook Air, but slightly smaller and doesn't feel chunky. The battery even kind-of works, so it's begging for a road trip somewhere.